1. Field of the Invention
This invention relates to monitor method and apparatus for overlay alignment of a stepper and particularly to a stepper and a reticle means used in exposing process for semiconductor manufacturing process to perform real time overlay alignment monitor when exposing a first layer pattern on a wafer.
2. Description of the Prior Art
In manufacturing process of integrated circuit (IC), one of the frequently used operations is to transfer circuit layout pattern to a wafer by means of microphoto techniques. One of the techniques is to make the circuit layout as a pattern on a reticle and uses a stepper to expose a wafer covered with a photoresist through the reticle in a stepping and repeating manner. The stepping and repeating manner means the reticle is being moved at a predetermined distance at a time and stop, and then exposing operation is performed. After that the reticle is moved again to repeat another cycle of operation set forth above until the circuit layout pattern has been transferred to the whole wafer surface.
In the IC manufacturing process, many layers of circuit layout are formed overlayed in the wafer at predetermined locations. The accuracy of overlaying greatly affects the IC electricity property, stability, reliability, and the yield of IC production. Thus enhancing microphoto technique is a heavily focused task in IC manufacturing industry. A lot of references regard "Pattern Alignment" technique have been disclosed. The "Pattern Alignment" technique includes mainly "Reticle Mark Alignment" and "Process Automatic Alignment" techniques.
According to the "Reticle Mark Alignment" technique, a plurality of Marks are provided on the reticle. The marks are transferred to the wafer through exposing (micro photo) process of a designated layer. When proceeding the exposing process of the reticle pattern of a subsequent layer, the stepper will locate or recognize the marks of a previous layer, and align the marks of an upper layer against a lower layer.
The "Process Automatic Alignment" technique forms a "Shield Layer" made of selected material (such as nitrous silicon) in a selected pattern on the wafer surface. This "Shield Layer" becomes a mask for the subsequent process and automatically aligns with the pattern of the previous layer. It thus saves at least one process step of spreading photoresist and reticle exposing when forming and self-aligning an upper layer circuit against a low layer circuit. Nevertheless all the techniques set forth above mainly focus how to align "an upper layer pattern" against "a lower layer pattern". They rarely address the overlaying accuracy of exposing pattern of the "initial layer" when using the stepper. It is generally known that the alignment accuracy of the initial layer directly affects the alignment accuracy of subsequent layers. Although the "Reticle Mark Alignment" technique may automatically provides positioning alignment function within a small error range, it does not adequately cover the initial pattern alignment. The accuracy of the initial layer alignment thus becomes a problem and will increase positioning error of subsequent pattern overlaying.
Conventional method of using the stepper for enhancing pattern overlay accuracy for the initial layer mainly address two areas: one is to enhance stepper precision to get better exposing accuracy. Another one is to perform periodical maintenance and parameter adjustment of the stepper. A "maintenance reticle" is placed in the stepper periodically (such as once in a number of weeks or months) for exposing a testing wafer to get the overlay error value. Then software parameters in the stepper are changed to correct the overlay error. The conventional method set forth above has the following disadvantages:
1. Periodical Maintenance (PM) approach does not meet "Real-Time Monitor" requirement in the production line. The "Maintenance Reticle" usually uses standard size and pattern which do not fit production of different size and pattern reticle. The alignment accuracy is thus not satisfactory. PA1 2. The adverse effect of lower accuracy of subsequent layers resulting from the alignment error of the "Initial Layer" cannot be totally eliminated. The alignment error of the initial layer comes from the moving error of the stepper or angular skew when placing the reticle in the stepper. That error deviation cannot be detected real time. Hence the positioning accuracy of subsequent layers based on the initial layer cannot be better than the initial layer. In most cases, the alignment error in the subsequent layers will aggregate and becomes worse. This phenomenon makes production and quality control very difficult and complicated, and can easily drag down the yield. PA1 3. "Periodical Maintenance" usually cannot meet production requirements, particularly when a lot of equipments and facilitates from different vendors are involved. They may have different maintenance requirements and cycle, and are difficult to schedule. Prolong downtime resulting from maintenance causes yield drop and increases production cost. PA1 4. Overlay alignment of mix and match among same type of equipments is difficult. Conventional method uses maintenance reticle of standard dimension and pattern. When there is need for mix and match overlay alignment between equipments for different size or pattern of reticle, it is very difficult to get the accuracy desired. PA1 a. Forming a rectangular-shaped first stepping exposing pattern on a base member through the stepper. The first stepping exposing pattern includes at least a first align mark and a second align mark located respectively at a center point of two opposite sides of the first stepping exposing pattern. The first and second align marks define a first straight line crossing the center line of the first stepping exposing pattern. PA1 b. Forming a second stepping exposing pattern on the base member by means of the stepper at a selected distance (.DELTA.X) from the first stepping exposing pattern along a second straight line. The second stepping exposing pattern is substantially same as the first stepping exposing pattern and also has a first align mark and a second align mark. The moving distance of the second stepping exposing pattern is such that the first align mark of the second stepping exposing pattern overlays with the second alignment mark of the first stepping exposing pattern. Defining a third straight line normal to the second straight line. PA1 c. Locating the center point of the first align mark of the second stepping exposing pattern. Projecting the center point on the first straight line along the third straight line. The projection distance is .DELTA.Y. PA1 d. Getting an angle based on the following equation: ##EQU1## Where .theta. is the angle formed by the first and second straight line. It is also the angular deviation when the stepper overlays the pattern.